Our AUV mapping and ROV diving operations associated with MBARI’s northern
expedition this summer are focused on investigatingknown gas vents,
chemosynthetic biological communities (CBC), and gas hydrate bearing sites along
the North American margin. The plans for the expedition involve collecting a
series of AUV mapping surveys from the R/V Zephyr at four sites. This data will
then be used to direct Doc Ricketts dives to ground truth the mapping surveys
and to help direct the design of other experiments and installations. Among
sites include the two summits of Hydrate Ridge
(Oregon), Bullseye Vent (Canada), and Barkley Canyon (Canada). These sites
continue to be targets for exploration and experimentation by researchers at
MBARI and many other institutions.
The installation of a seafloor observatory to monitor natural variations
in gas and gas hydrate dynamics and to conduct perturbation experiments remains
one of the most supported concepts within the various Ocean Observing Initiative
efforts. The planned sites for the installation of gas hydrate observatories, as
part of NEPTUNE Canada (including future capacity to include an IODP CORK), are
Bullseye vent and Barkley
Canyon. The other candidate for a cable-connected system is Hydrate Ridge. While
an initial deployment of instruments is planned for NEPTUNE Canada in 2009,
detailed high-resolution mapping is still lacking in parts of these areas and is
necessary for the long-term planning for expansion of hydrate observatory
infrastructure. This can be best (and perhaps only) provided with a system like
the MBARI mapping AUV.
A particular focus of this expedition is the nature and origin of the
micro-topography associated with these gas-rich seafloor environments. Visual
observations in many such environments have already shown that the seafloor
where CBCs are on the surface and gas hydrate are believed to be in the near
subsurface is commonly associated with seafloor blisters or mounds of various
sizes, shallow depressions (e.g., ~3 m wide and ~1 m deep), as well as an
occasional small up-turned ridge of truncated strata. The origin of this
topography remains unexplained, yet is potentially critical for assessing the
extent to which gas hydrates are geo-hazards and to understand how gas hydrates
may shape the seafloor. Moreover, up-to-date maps that document these features
do not exist, largely because of the limitations of previously available
surveying tools. We are interested in documenting these features and evaluating
two possible processes that may play a role in creating these features:
(a) The area around these vents may be experiencing seafloor modification
because
of gas hydrate growth. Subsurface gas hydrate formation may inflate the seafloor
producing pingo-like features that range from 1 to 10 m in height and width.
When sections of seafloor contain >92% gas hydrate by volume they are buoyant in
seawater.
Thus, gas hydrate-bearing sediment masses may break loose from the seafloor and
float away, leaving depressions on the seafloor (Paull et al., 1996). Repeat
visits with an ROV or HOV to vent sites in the Gulf of Mexico (Roberts et al.,
2001) and Barkley Canyon
(Ross Chapman, personal communication) have shown that gas hydrate mounds
observed in previous years have disappeared, apparently leaving holes.
Presently, it is unknown whether these changes are of a magnitude that will be
within the resolution of the
mapping tools on the AUV.
(b) We will also focus on documenting the extent and importance of seafloor
erosion on the flanks of these structures. Occurrences of carbonate-cemented
nodules and concretions exposed on the seafloor that contain cements with light
carbon isotopes, indicating a contribution of methane-derived carbon, are
commonly interpreted to be indicators of seafloor fluid venting and occur around
all these vent systems. However, the fine-grained clastic sediments that
typically host these carbonates indicate that many, if not most, of these
carbonates were formed within the subsurface, presumably associated with
anaerobic oxidation of methane at the sulfate-methane transition zone. We
hypothesize that the widespread occurrence of these carbonates on the seafloor
may result from persistent seafloor erosion, which preferentially removes the
uncemented sediments and leaves the more resistant, cemented sediments standing
in relief. Combined multibeam bathymetry, sub-bottom profiling, and side-scan
sonar data obtained by the mapping AUV will provide a basic context to
understand the nature of these systems and the broad area around them. ROV
ground truth sampling also will be required to evaluate these questions.
An MBARI mapping AUV survey was conducted at Barkley Canyon in 2007.
However, we are interested in returning, in part, to assess whether any changes
in the morphology can be quantified and to refine and enlarge the existing
survey coverage.
The existing schedule allows for up to 7 18-hour long AUV mapping survey
missions. We have scheduled this as 2 at Hydrate Ridge (44° 34’N 125° 09’W); 2
at Bullseye vent (48° 41’ N 126° 51’W); 2 at Barkley Canyon (48° 19’N, 126° 04
W); and one as a contingency. These would be followed by a 10-day Western Flyer
expedition out of Newport, Oregon, to ground truth these survey areas. We intend
to work with Ross Chapman (University of Victoria) on the Barkley Canyon surveys
and Michael Riedel (McGill University) on the Bullseye vent surveys, both as a
scientific collaboration and to assist in plans for the installation of Neptune
observatories.
The day-by-day plan for this trip will be highly dependant on the weather
and the results of the AUV survey’s. However, it is likely that we will dive on
Hydrate Ridge the first day, as it will make logistical sense. On this dive we
will focus on sampling up the side of the feature called the Pinnacle which has
that has been described as an upward growing chemoherm (e.g., Teicher et al.,
2005), but we hypothesis it is an erosional feature (Paull and Ussler, 2008).
At Barkley canyon we anticipate picking up some gear of Ross Chapman’s. At
Bulleye Vent we expect to deploy an Osmo-sampler probe that is to be stuck into
the sediment. This probe is designed by Laura Lapham. We believe that this is of
a size that will allow it to be carried on the Doc Ricketts.